3uO 



THE AMERICAN NATURALIST 



[Vol. LII 



As regards the measurable characters which differen- 

 tiate the parent races, some light upon their behavior in 

 hybridization may be derived from the data in Table IX. 



Here, too, one fact seems plain, despite the small num- 

 ber of individuals, and the modifications which these mice 

 share with cage-born mice in general. This is the inter- 

 mediate condition of the hybrids, both and F 2 , as re- 

 gards tail length, foot length and tail-stripe. The ear 

 measurements have not been introduced into these com- 

 parisons, since the parent races do not differ in this re- 



As regards variability, it is seen that the standard de- 

 viations of the F 2 generation are all three larger than 

 those of the F 1? and the differences seem great enough to 

 be of possible statistical significance. I have, however, 

 tabulated the frequencies of the various values for these 

 three character.- (not reproduced here), and I find that 

 the probable explanation of this increase of variability in 

 the F 2 generation is an increase in the amount of abnor- 

 mality in the latter. This is to be inferred from the fact 

 that the extension in range is chiefly in the direction of 

 the lower values, while in two of the three cases the up- 

 permost figures actually fall below those of the Fj. Xow 

 the abnormal influences of captivity operate by decreas- 

 ing (oftentimes considerably) the values for these very 

 characters. As further evidence for this interpretation 

 is the fact that the average body size of the F 2 mice is 

 here less than that of the preceding generation. 



A considerable number of back crosses have been ob- 

 tained between each of the hybrid combinations above 

 discussed and one or both of the parent races. The num- 

 ber of individuals in any one series is, however, small, so 

 that it is hardly worth while to deal with the results of 

 these crosses here. They afford as little evidence of 

 complete segregation of the subspecific color types as do 

 the hybrids previously considered. 



